The ABC System - an early CTC

It was realized very early that the key to increasing the capacity of an
American single-track railroad was the elimination of train orders, running
trains by signal indication under a centralized control, which would supersede
the superiority of trains. One way to do this was certainly known, the British
manual block system. However, the expense for signalmen, block stations,
interlocking, signals, and communications was prohibitive, except for those
busy double-track lines that already had something better. In 1907, the
Northern Pacific came up with a cheap method that only used existing
facilities, operators and train order signals, to accomplish this objective.

In this system, called the "ABC" system, the operators at each end of a
block and the train dispatcher cooperated to signal trains to stop or proceed
by means of the block signal alone. Since the agreement of three minds was
necessary to move trains, the system was regarded as safe enough that
superiority of trains could be abolished.

All the operators, and the dispatcher, were on the same wire, so everyone
was always aware of what was happening. When a train accepted a clear signal,
it was also handed up a clearance card while moving at 25 mph or so, using a
hoop. When trains were to meet, they were notified by orders on the card at the
station in the rear that this was to occur at the next station. They would then
approach this station with caution, taking siding or holding the main line as
directed on the card. This was necessary, because the train order signals were
not properly located to protect such a movement.

The system was first used near Spokane, Washington, and then was extended
over some 600 miles of line as far east as Billings, Montana. The system is
reported to have accelerated freight movement by 20 to 25 per cent, by
eliminating the time required to prepare and deliver orders in the usual
manner.

However, the system seems to have been somewhat half-baked. There were some
collisions when freight trains did not agree on who was to hold the main track
at meets. Passenger trains were seriously delayed, it seems, and this was cited
as the main reason for abandoning the ABC System after a few years. By 1910 it
was gone, replaced by the normal manual block, and by automatic signals between
Billings and Livingston.

The ABC System must have been a severe strain on the dispatcher, since he
could no longer think out problems and compose orders to solve them at one
time, but had to react as the trains were moving, and make quick decisions. The
operators were there as a check, but a good dispatcher does not depend on his
operators for safety. These concerns could have been relieved by use of train
staff, but this was never done. The small cost of the train staff was more than
the zero cost of the ABC system, which doomed it. The ABC System was the first
significant attempt to institute centralized control of trains in the United
States over long distances.